Typically, force limiters employed in safety belt systems are intended to limit the maximum force acting on the safety belt to a value which is uncritical for the vehicle occupant restrained by the safety belt. When this value is reached in the safety belt, a relative rotation or twist between shaft and the anchoring part the force limiter makes available an additional length of belt webbing, which may be used for additional deceleration displacement and prevents any further increase in the force acting in the safety belt.
Such a force limiter may be employed in various different positions in the safety belt system. In recent times belt retractors are being fitted with such a force limiter, which when the predetermined maximum force in the belt webbing is exceeded, permits twist of the belt drum in relation to the belt retractor in the belt webbing pay-off direction with the result that belt webbing is released. For this purpose it is more especially possible to utilize a torsion rod connected on the one hand with the belt drum and on the other hand with the belt retractor. When a certain torque as set by the dimension of the torsion rod is reached, the belt drum may be twisted in relation to the belt retractor with an elastic and/or plastic deformation of the torsion rod. A disadvantage of such a force limiter is however that the characteristic of such a force limiter, that is to say the function of torque versus angle of twist between the belt drum and the belt retractor or the function of belt webbing force versus paid off belt webbing, may only be modified to a small degree. Presently the only practice is to change the effective length of the torsion rod, for example by mechanically or electromechanically changing the effective length of the torsion rod during operation, in order to modify the characteristic of the force limiter, for instance for adaptation to the weight of a vehicle occupant. However this only allows a change to be made in the spring constant of the torsion rod; the characteristic as a whole can not be influenced.